Release of deuterated nonenal during beer aging from labeled precursors synthesized in the boiling kettle

The use of labeled nonenal enabled the demonstration that the appearance of the cardboard flavor in finished beer comes from lipid auto-oxidation duri ng wort boiling and not from lipoxygenasic activity during mashing. Free tr ans-2-nonenal produced by linoleic acid auto-oxidation in the kettle disapp ears, owing to retention by wort amino acids and proteins. This binding lin kage protects trans-2-nonenal from yeast reduction but is reversible, allow ing release of the compound at lower pH during aging. Labeled trans-2-nonen al is detected after aging when deuterated precursors form in the boiling k ettle. The amount of alkenal released correlates with the concentration of reversible associations in the pitching wort. This work brings new illumina tion to the formation of trans-2-nonenal and overturns many previous hypoth eses. It also explains why a reduction in the beer pH intensifies the cardb oard flavor.